As semiconductor integrated circuit chips become more multi-functional and highly integrated, the chips include more bonding pads (or terminal pads), and thus packages for the chips have more external terminals (or leads). When a conventional package having leads along the perimeter of the package must, accommodate a large number of electrical connection points, the footprint of the package increases. However, a goal in many electronic systems is to minimize an overall size of the systems. Thus, to accommodate a large number of pins without increasing the footprint of the package, either pin pitch (or lead pitch) of the package must decrease and/or integrated circuit dice need to be stacked within a single package (a stacked package). However, a pin pitch of less than about 0.4 mm gives rise to many technical concerns. For example, trimming of a package having a pin pitch less than 0.4 mm requires expensive trimming tools, and the leads are prone to bending during handling of the package. In addition, surface-mounting of such packages demands a costly and complicated surface-mounting process due to a required critical alignment step.
The stacked package, however, may be used to increase a surface, density within the same footprint of the package. Such stacked configurations are known in the art.
In a standard integrated circuit (e.g., a circuit formed on bulk silicon), an IC designer will include one or more bond pads on the top layer design, of the IC. The bond pad is electrically connected to the substrate by wire-bonding the IC to a ground potential. In order for this scheme to function properly, there must be a ground bond pad an the IC.
However, many advanced semiconductor integrated circuit devices are constructed, on substrates where the integrated circuit device is fabricated on a top layer that is electrically insulated from a lower portion (base) of the substrate. These substrate types include separation by implantation of oxygen (SIMOX) and silicon-on-insulator (SOI). In these cases, there is no electrical connection from the integrated circuit circuitry on an uppermost portion of the insulated substrate to the lower portion of the substrate. Thus, in SOI technology, the integrated, circuit (along with any bond pad) is fabricated into a top layer of the SOI material. The insulator layer prevents designing, a substrate-ground bond pad into the IC design for an SOI device. Thus, grounding a ground bond pad with a wire bond will not electrically ground the insulated base portion.
FIG. 1 is an elevation view of a prior art stacked die package 100. The stacked die package 100 includes a die-attach paddle 101, a plurality of package pads 103, a silicon, integrated circuit die 105, and an SOI integrated circuit die 107. The silicon integrated circuit die 105 is adhered to the die-attach paddle 101 by conductive epoxy 109. The SOI integrated circuit die 107, in turn, is adhered to the silicon integrated circuit die 105 by a non-conductive epoxy 111. (Note that there is no reason to use conductive epoxy for adhering the SOI die 107 since no electrical contact can be made with the fabricated circuitry on the top layer of the SOI die 107.) A plurality of wire bond pads 119 are formed on both the silicon die 105 and the SOI die 107. A plurality of SOT wire bonds 113 and a plurality of silicon wire bonds 115 electrically connect the SOI die 107 and silicon die 105 respectively to the plurality of package pads 103. After all electrical connections are formed, an encapsulant layer 121 protects the integrated circuit dice 105, 107 and the plurality of wire bonds 113, 115.
The presence of an insulator layer 123 on the SOI die 107 electrically isolates an SOI base 125 from the circuitry formed on the top layer. Additionally, the SOI die 107 is mounted over a passivation layer (not shown) of the silicon die 105 thereby further preventing grounding of the SOI base.
Therefore, what is needed is a simple and economical means of electrically grounding the base in integrated circuit production materials which contain an insulative layer.